Optical switches based on induced optical loss

ABSTRACT

An optical switch device includes a first semiconductor structure configured to operate as a first waveguide and a second semiconductor structure configured to operate as a second waveguide. The second semiconductor structure is located above or below the first semiconductor structure and separated from the first semiconductor structure. The second semiconductor structure includes a first portion having a first width and a second portion having a width different from the first width and located on the first portion. The first portion is located between a first doped region and a second doped region.

TECHNICAL FIELD

This relates generally to photonic devices and, more specifically, tooptical switch devices.

BACKGROUND

Optical switch devices are used in various optical applications, such asoptical communications. High speed optical switch devices often includephase shifters. In such optical switch devices, phase shifts (byelectro-optic effects or magneto-optic effects) in one or more opticalwaveguides are adjusted to output different portions of input light fromthe optical waveguides. However, such optical switch devices requirecareful tuning of the phase shifts to achieve high contrast between onand off states.

SUMMARY

In accordance with some embodiments, an optical switch device includes afirst semiconductor structure configured to operate as a firstwaveguide; and a second semiconductor structure configured to operate asa second waveguide. The second semiconductor structure is located aboveor below the first semiconductor structure and separated from the firstsemiconductor structure. The second semiconductor structure includes afirst portion having a first width and a second portion having a widthdifferent from the first width and located on the first portion. Thefirst portion is located between a first doped region and a second dopedregion.

In some embodiments, the first semiconductor structure and the secondsemiconductor structure are configured to couple light propagating inthe first waveguide to the second waveguide while a first voltagesatisfying a first voltage condition is applied between the first dopedregion and the second doped region. The first semiconductor structureand the second semiconductor structure are configured to forego couplingof the light propagating in the first waveguide to the second waveguidewhile a second voltage satisfying a second voltage condition is appliedbetween the first doped region and the second doped region.

In some embodiments, the second semiconductor structure has a firstcarrier density while the first voltage is applied between the firstdoped region and the second doped region. The second semiconductorstructure has a second carrier density that is greater than the firstcarrier density by a factor of at least 100 while the second voltage isapplied between the first doped region and the second doped region.

In some embodiments, the first portion has a first absorption propertywhile the first voltage is applied between the first doped region andthe second doped region and a second absorption property that isdifferent from the first absorption property while the second voltage isapplied between the first doped region and the second doped region.

In some embodiments, the second portion includes a plurality of firstsections having a second width interleaved by a plurality of secondsections having a third width different from the second width.

In some embodiments, the second portion includes a plurality of firstsections having a first thickness interleaved by a plurality of secondsections having a second thickness different from the first thickness.

In some embodiments, each first section of the plurality of firstsections has a first length; and each second section of the plurality ofsecond sections has a second length that is different from the firstlength.

In some embodiments, the first semiconductor structure is made of afirst semiconductor material having a first index of refraction; and thesecond semiconductor structure is made of a second semiconductormaterial having a second index of refraction that is different from thefirst index of refraction.

In some embodiments, the first doped region is doped with donor dopants,and the second doped region is doped with acceptor dopants.

In some embodiments, one of the first waveguide and the second waveguideis connected to an input port of the optical switch device for receivinglight. The first waveguide is connected to a first output port of theoptical switch device. The second waveguide is connected to a secondoutput port of the optical switch device that is different from thefirst output port of the optical switch device.

In accordance with some embodiments, an optical switch device includes afirst semiconductor structure configured to operate as a first waveguideand a second semiconductor structure configured to operate as a secondwaveguide. The second semiconductor structure is located above or belowthe first semiconductor structure and separated from the firstsemiconductor structure. The second semiconductor structure includes aportion of a first doped region doped with dopants of a first type and aportion of a second doped region doped with dopants of a second typethat is different from the dopants of the first type.

In some embodiments, the second semiconductor structure includes aplurality of first-cross-section regions interleaved by a plurality ofsecond-cross-section regions along the direction of the secondwaveguide.

In some embodiments, each first-cross-section region of the plurality offirst-cross-section regions has a first width, and eachsecond-cross-section region of the plurality of second-cross-sectionregions has a second width that is different from the first width.

In some embodiments, each first-cross-section region of the plurality offirst-cross-section regions has a first thickness, and eachsecond-cross-section region of the plurality of second-cross-sectionregions has a second thickness that is different from the firstthickness.

In some embodiments, the plurality of first-cross-section regionsincludes first, second, and third regions and the plurality ofsecond-cross-section regions includes fourth and fifth regions. Thefirst, second, and third regions are interleaved by the fourth and fifthregions so that the fourth region is located between the first andsecond regions and the fifth region is located between the second andthird regions. The optical switch device also includes (i) a pluralityof regions doped with the dopants of the first type, including the firstdoped region and a third doped region, and (ii) a plurality of regionsdoped with the dopants of the second type, including the second dopedregion and a fourth doped region. The first doped region and the seconddoped region include the first, fourth, and second regions. The thirddoped region and the fourth doped region include the second, fifth, andthird regions.

In some embodiments, the plurality of first-cross-section regionsincludes a sixth region and the plurality of second-cross-sectionregions includes a seventh region, the seventh region being locatedbetween the third region and the sixth region. The plurality of regionsdoped with dopants of the first type also includes a fifth doped regionand the plurality of regions doped with dopants of the second type alsoincludes a sixth doped region. The fifth doped region and the sixthdoped region include the third, seventh, and sixth regions. The fourthdoped region is located between the first and fifth doped regions, andthe third doped region is located between the second and sixth dopedregions.

In some embodiments, the first doped region is in contact with thesecond doped region, and the third doped region is in contact with thefourth doped region.

In some embodiments, the third doped region is separated from the firstdoped region and the second doped region, and the fourth doped region isseparated from the first doped region and the second doped region.

In some embodiments, the first semiconductor structure is made of afirst semiconductor material having a first index of refraction, and thesecond semiconductor structure is made of a second semiconductormaterial having a second index of refraction that is different from thefirst index of refraction.

In accordance with some embodiments, a method includes transmittinglight into the first semiconductor structure of any optical switchdevice described herein while a first voltage satisfying a first voltagecondition is applied between the first doped region and the second dopedregion for coupling the light from the first waveguide to the secondwaveguide.

In some embodiments, the method also includes, prior to, or subsequentto, coupling the light from the first waveguide to the second waveguide,transmitting the light into the first semiconductor structure while asecond voltage satisfying a second voltage condition different from thefirst voltage condition is applied between the first doped region andthe second doped region for propagating the light within the firstwaveguide without coupling the light from the first waveguide to thesecond waveguide.

In some embodiments, the second semiconductor structure has a firstcarrier density while the first voltage is applied between the firstdoped region and the second doped region, and the second semiconductorstructure has a second carrier density that is greater than the firstcarrier density by a factor of at least 100 while the second voltage isapplied between the first doped region and the second doped region.

In some embodiments, the light is coupled from the first waveguide tothe second waveguide while the optical switch device is at a temperaturebetween 40 Kelvin and 200 Kelvin.

In some embodiments, applying the second voltage between the first dopedregion and the second doped region while the optical switch device is ata temperature less than 40 Kelvin allows coupling of the light from thefirst waveguide to the second waveguide.

In accordance with some embodiments, an optical switch device includes afirst waveguide including a first portion coupled with a first regiondoped with first dopants and a second portion coupled with a secondregion doped with second dopants. The optical switch device alsoincludes a second waveguide located adjacent to the first waveguide forcoupling light from the first waveguide to the second waveguide. Thesecond waveguide includes a third portion coupled with a third regiondoped with first dopants and a fourth portion coupled with a fourthregion doped with second dopants. The first portion is located adjacentto the third portion and the second portion is located adjacent to thefourth portion.

In some embodiments, the first waveguide includes a plurality of firstportions coupled with regions doped with the first dopants and aplurality of second portions coupled with regions doped with the seconddopants. The plurality of first portions is interleaved with theplurality of second portions. The second waveguide includes a pluralityof third portions coupled with regions doped with the first dopants anda plurality of fourth portions coupled with regions doped with thesecond dopants. The plurality of third portions is interleaved with theplurality of fourth portions.

In some embodiments, the first region and the second region areconfigured to receive a voltage satisfying a first voltage conditionbetween the first region and the second region, and the third region andthe fourth region are not configured to receive a voltage satisfying thefirst voltage condition between the third region and the fourth region.

In some embodiments, the optical switch device also includes a resistiveheater located adjacent to the first waveguide and the second waveguidefor changing a temperature of the first waveguide and the secondwaveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the various described embodiments,reference should be made to the Detailed Description below, inconjunction with the following drawings in which like reference numeralsrefer to corresponding parts throughout the figures.

FIGS. 1A and 1B are partial plan views of an optical switch deviceillustrating optical paths of light propagating within the opticalswitch device in accordance with some embodiments.

FIG. 1C illustrates example intensity values of light within thewaveguides shown in FIGS. 1A and 1B for different real components of apropagation constant of a waveguide.

FIG. 1D illustrates example intensity values of light within thewaveguides shown in FIGS. 1A and 1B for different imaginary componentsof a propagation constant of a waveguide.

FIG. 1E illustrates example intensity values of light within thewaveguides shown in FIGS. 1A and 1B for different phases of apropagation constant.

FIG. 1F illustrates the power of light remaining in a first waveguidefor selected carrier densities in a second waveguide, for threedifferent example configurations.

FIG. 1G illustrates example free carrier densities needed to achieve acertain efficiency in the optical switch device.

FIGS. 2A and 2B are enlarged views of a coupling region of the opticalswitch device shown in FIGS. 1A and 1B in accordance with someembodiments.

FIG. 2C is a cross-sectional view of the coupling region shown in FIG.2B.

FIG. 2D is an enlarged view of the coupling region with multipleinstances of the structure shown in FIG. 2A.

FIGS. 3A and 3B are partial plan views of an optical switch device inaccordance with some embodiments, in which optical waveguides arestacked vertically.

FIGS. 3C and 3D are cross-sectional views of the optical switch deviceshown in FIG. 3A.

FIG. 4A is an enlarged view of a coupling region of the optical switchdevice shown in FIG. 3A in accordance with some embodiments.

FIG. 4B is a cross-sectional view of the coupling region shown in FIG.4A.

FIG. 5A is an enlarged view of the coupling region of the optical switchdevice shown in FIG. 3A in accordance with some embodiments.

FIG. 5B is a cross-sectional view of the coupling region shown in FIG.5A.

FIGS. 6A and 6E are enlarged views of the coupling region of the opticalswitch device shown in FIG. 3A in accordance with yet some otherembodiments.

FIGS. 6B-6D are cross-sectional views of the coupling region shown inFIG. 6A.

FIG. 6F is a cross-sectional view of the coupling region shown in FIG.6E.

FIG. 7A is an enlarged view of the coupling region of the optical switchdevice shown in FIG. 3A in accordance with some embodiments.

FIG. 7B is a cross-sectional view of the coupling region shown in FIG.7A.

FIGS. 8A and 8B illustrate optical switch devices in accordance withsome embodiments, in which a number of input ports is different from anumber of output ports.

FIG. 9 is a flowchart illustrating a method of operating an opticalswitch device in accordance with some embodiments.

Like reference numerals refer to corresponding parts throughout theseveral views of the drawings. The drawings may not be drawn to scaleunless stated otherwise.

DETAILED DESCRIPTION

Deficiencies and other problems associated with optical switch devicesincluding phase shifters are reduced or eliminated by the optical switchdevices and methods described herein. The disclosed optical switchdevices and methods utilize modulation of absorption properties of oneor more optical waveguides for switching operations. Such modulation ofabsorption properties can allow the optical switch devices to operate asa binary switch (e.g., the optical switch device is in an “off” statewhile the modulated absorption property of a particular opticalwaveguide is above a threshold absorption value and the optical switchdevice is in an “on” state while the modulated absorption property ofthe particular optical waveguide is below the threshold absorptionvalue), thereby eliminating the need for monitoring and tuning phaseshifts induced by phase shifters and enabling compact and robust opticalswitch devices.

In addition, the disclosed optical switch devices may include structuresthat facilitate large modulation of the absorption properties of the oneor more optical waveguides. This further reduces the size of the opticalswitch devices and also eliminates the need for a high voltage source.

In some cases, the optical switch devices may include multi-mode opticalwaveguides, which reduces the optical loss associated with interactionbetween light propagating within an optical waveguide and the side wallsof the optical waveguide, which, in turn, reduces the loss of thetransmitted light.

Reference will now be made in detail to embodiments, examples of whichare illustrated in the accompanying drawings. In the following detaileddescription, numerous specific details are set forth in order to providea thorough understanding of the various described embodiments. However,it will be apparent to one of ordinary skill in the art that the variousdescribed embodiments may be practiced without these specific details.In other instances, well-known methods, procedures, components,circuits, and networks have not been described in detail so as not tounnecessarily obscure aspects of the embodiments.

It will also be understood that, although the terms first, second, etc.are, in some instances, used herein to describe various elements, theseelements should not be limited by these terms. These terms are used onlyto distinguish one element from another. For example, a first waveguidecould be termed a second waveguide, and, similarly, a second waveguidecould be termed a first waveguide, without departing from the scope ofthe various described embodiments. The first waveguide and the secondwaveguide are both waveguides, but they are not the same waveguide. Inanother example, a first semiconductor structure could be termed asecond semiconductor structure, and, similarly, a second semiconductorstructure could be termed a first semiconductor structure, withoutdeparting from the scope of the various described embodiments. The firstsemiconductor structure and the second semiconductor structure are bothsemiconductor structures, but they are not the same semiconductorstructure.

FIGS. 1A and 1B illustrate partial plan views of optical paths of lightin an optical switch device 100 in accordance with some embodiments.

The optical switch device 100 includes a first waveguide 150 and asecond waveguide 152 that is distinct and separate from the firstwaveguide 150. A portion 160-A of the first waveguide 150 is opticallycoupled to a portion 170-A of the second waveguide 152 in a couplingregion 200 so that light 110 propagating in the first waveguide 150 istransferred to the second waveguide 152 through evanescent coupling asshown in FIG. 1A while the absorption property of the coupled portion170-A of the second waveguide 152 is below a threshold absorption value.

A coupling efficiency between the first waveguide 150 and the secondwaveguide 152 is determined based on the length of the portions 160-Aand 170-A, in addition to the real and imaginary refractive index of thefirst waveguide 150, the real and imaginary refractive index of thesecond waveguide 152, the width and height of the first waveguide 150,the width and height of the second waveguide 152, the real and imaginaryrefractive index of the material located between the first waveguide 150and the second waveguide 152, and the distance between the firstwaveguide 150 and the second waveguide 152. In some embodiments, thedistance between the first waveguide 150 and the second waveguide 152 isselected to increase the coupling efficiency between the first waveguide150 and the second waveguide 152. In some embodiments, the distancebetween the first waveguide 150 and the second waveguide 152 is lessthan 200 nm, between 100 nm and 300 nm, between 200 nm and 400 nm,between 300 nm and 500 nm, or greater than 400 nm, although otherdistances may be used, depending on the other dimensions of the firstwaveguide 150 and the second waveguide 152 and selection of thematerials for the first waveguide 150 and the second waveguide 152. Insome embodiments, the length of the coupled portion 160-A of the firstwaveguide 150 or the coupled portion 170-A of the second waveguide 152(i.e., the distance between lines 102-1 and 102-2) is selected so thatthe coupling efficiency between the first waveguide 150 and the secondwaveguide 152 (for a given set of parameters for the refractive index ofthe first waveguide 150, the refractive index of the second waveguide152, the width and height of the first waveguide 150, the width andheight of the second waveguide 152, the refractive index of the materiallocated between the first waveguide 150 and the second waveguide 152,and the distance between the first waveguide 150 and the secondwaveguide 152) is close to 100% (e.g., greater than 99%). In someembodiments, the length of the coupled portion 160-A of the firstwaveguide 150 or the coupled portion 170-A of the second waveguide 152is between 5 μm and 200 μm but other lengths are possible withoutdeparting from the scope of the present disclosure.

FIG. 1B illustrates that the absorption property of the portion 170-A ofthe second waveguide 152 in the coupling region is increased above thethreshold absorption value so that the light 110 propagating in thefirst waveguide 150 remains within the first waveguide 150 withouttransferring to the second waveguide 152.

Without limiting the scope of claims, this can be described with anumerical model, in which the amplitudes of light within the firstwaveguide 150 and the second waveguide 152 satisfy the following:∂_(x) a ₁(z)=iκa ₂(z)∂_(x) a ₂(z)=iκa ₁(z)+iΔβκa ₂(z)where a₁ and a₂ are the amplitudes of light within the first waveguide150 and the second waveguide 152, κ is a coupling constant for theevanescent coupling between the first waveguide 150 and the secondwaveguide 152, and Δβ represents a tunable shift in the propagationconstant of the second waveguide 152, with both real part (associatedwith the propagation phase) and imaginary part (associated with theloss).

FIG. 1C illustrates the intensity of the light within the firstwaveguide 150 (shown on the left-hand side) and the intensity of thelight within the second waveguide 152 (shown on the right-hand side)based on changing the real part (associated with the propagation phase)of the propagation constant of the second waveguide 152. As shown inFIG. 1C, by adjusting the real component of Δβ/κ from 0 to 10, theintensity of light transferred from the first waveguide 150 to thesecond waveguide 152 after a propagation length z=π/(2κ) changes fromnear 100% to near 0%. Thus, the device 100 can operate as an opticalswitch by changing the real component of Δβ.

FIG. 1D illustrates the intensity of the light within the firstwaveguide 150 (shown in the left-hand side) and the intensity of thelight within the second waveguide 152 (shown on the right-hand side)based on changing the imaginary part (associated with the optical loss)of the propagation constant of the second waveguide 152. As shown inFIG. 1D, by adjusting the imaginary component of Δβ/κ from 0 to 10, theintensity of light transferred from the first waveguide 150 to thesecond waveguide 152 after the propagation length z=π/(2κ) changes fromnear 100% to near 0%. Thus, the device 100 can also operate as anoptical switch by changing the imaginary component of Δβ.

FIG. 1E illustrates the intensity of the light within the firstwaveguide 150 (shown in the left-hand side) and the intensity of thelight within the second waveguide 152 (shown on the right-hand side)based on changing |Δβ/κ|. As shown in FIG. 1E, when a ratio α betweenthe real imaginary component and the real component of Δβ is at least 5%(in this particular configuration), the intensity of the light withinthe first waveguide guide 150 and the intensity of the light within thesecond waveguide 152 remain relatively stable compared to the case wherethe imaginary component of Δβ is zero. For example, when α=5%, for arange where |Δβ/κ|>25, at least 99% of the light within the firstwaveguide 150 remains within the first waveguide 150 and less than 1% ofthe light within the first waveguide 150 is transferred to the secondwaveguide 152, regardless of the phase shift in ββ. Thus, the circuitsfor monitoring and adjusting the phase shift in Δβ can be simplified.

In some embodiments, a propagation constant of a waveguide is changed byadjusting a density of free carriers (e.g., electrons or holes) withinthe waveguide, which, in turn, changes the refractive index of thematerial constituting the waveguide. In particular, increasing thedensity of free carriers increases the absorption of light within thewaveguide at least in part due to the increased free carrier absorption.FIG. 1F illustrates simulation results showing the power of lightremaining in the first waveguide 150 for example carrier densities inthe second waveguide 152 ranging from 10¹⁷ cm⁻³ to 10¹⁹ cm⁻³, for threedifferent configurations (left: the gap between the first waveguide 150and the second waveguide 152 is 300 nm, middle: the gap between thefirst waveguide 150 and the second waveguide 152 is 400 nm, and right:the gap between the first waveguide 150 and the second waveguide 152 is450 nm). Waveguides 150 and 152 made of silicon and each having a widthof 500 nm and a thickness of 220 nm were used in the numericalsimulation. When the carrier density of the second waveguide 152 is 10¹⁷cm⁻³, the intensity of light within the first waveguide 150 decreases tonear 0% as the propagation length increases (indicating that the lightis transferred from the first waveguide 150 to the second waveguide152), and when the carrier density of the second waveguide 152 is 10¹⁹cm⁻³, the intensity of light within the first waveguide 150 remains near100% regardless of the propagation length. Thus, changing the carrierdensity in a silicon waveguide from 10¹⁷ cm⁻³ to 10¹⁹ cm⁻³, for example,changes the operating mode of the optical switch device 100 from theevanescently coupled mode (as shown in FIG. 1A) to a non-coupled mode(as shown in FIG. 1B).

FIG. 1G illustrates the example free carrier densities needed to achievehigh efficiency (e.g., less than 0.1 dB loss) in the two states (the“on” state corresponding to the evanescently coupled mode and the “off”state corresponding to the non-coupled mode) of the optical switchdevice 100. The left chart indicates that the optical switch device 100in the “on” state has less than 0.1 dB loss (e.g., less than 0.1 dB ofthe light remains in the first waveguide 150) while the carrier densitywithin the second waveguide 152 remains less than approximately 4¹⁶ cm⁻³(for a 400 μm coupling length optical switch). The middle chartindicates that the optical switch device 100 in the “off” state has lessthan 0.1 dB loss (e.g., less than 0.1 dB of the light is transferred outof the first waveguide 150) while the carrier density within the secondwaveguide 152 remains greater than approximately 6¹⁸ cm⁻³ (for the 400μm coupling length optical switch). The schematic diagram shown on theright side of FIG. 1G illustrates that the optical switch device 100having a particular coupling length alternates between a low loss (e.g.,less than 0.1 dB loss) “on” state and a low loss “off” state by changingthe free carrier density within the second waveguide 152. In someimplementations, switching between the two switch states may requirechanging the free carrier density by 100 fold or more (e.g., 150 foldfrom 4¹⁶ cm⁻³ to 6¹⁸ cm⁻³) for example to maintain the high efficiency(e.g., less than 0.1 dB loss), although a person having ordinary skillin the art would understand that different free carrier densities ordifferent ratios of the free carrier densities may be used (e.g.,depending on the materials used in forming the first waveguide 150, thesecond waveguide 152, the material between the first waveguide 150 andthe second waveguide 152, their structures, and a loss tolerance).

Thus, the optical switch device 100 with a high efficiency may require ahigh spatial contrast in the free carrier density between the firstwaveguide 150 and the second waveguide 152 (e.g., the first waveguide150 has ˜10¹⁶ cm⁻³ carrier density or higher while the second, adjacentwaveguide 152 has ˜10¹⁹ cm⁻³ carrier density or lower during the “off”state of the optical switch device 100) and a high temporal contrast inthe free carrier density in the second waveguide 152 (e.g., the secondwaveguide 152 has ˜10¹⁶ cm⁻³ carrier density or higher while the opticalswitch is in the “on” state and the second waveguide 152 has ˜10¹⁹ cm⁻³carrier density or lower while the optical switch device 100 is in the“off” state).

In some cases, the density of free carriers (e.g., electrons or holes)within the waveguide is adjusted by utilizing the field-ionizationeffect. The field-ionization effect is beneficial when the opticalswitch device 100 is at a low temperature (e.g., below 100K, below 70K,below 40K, etc.). At room temperature, dopant atoms commonly used in thesemiconductor industry are ionized and contribute carriers to theconduction or valence band. “N-type” dopants, such as arsenic orphosphor in silicon, contribute an electron to the conduction band ofthe silicon, whereas “p-type” dopants, such as boron in silicon,contribute a hole to the valence band. If the ambient temperature islowered sufficiently, however, the dopant atoms retain their carriersand remain neutral. This is called “dopant freezeout.” This freezeoutoccurs at temperatures where kT (k=Boltzmann's constant) is smallrelative to the activation energy of the dopant, where the activationenergy corresponds to the energy difference between the dopant's defectlevel and the relevant band edge. The activation energy of commondopants (e.g. boron, arsenic, and phosphor) in silicon is ˜45-50 meV andcarrier freezeout starts at a temperature around approximately 100-200Kand becomes more significant at lower temperatures. However, if anelectric field is applied to frozen dopants, the dopants can be ionized,even at extremely low temperatures so that the ionized dopants cancontribute carriers to the conduction or valence band. In some cases,the electric field required to ionize common dopants in Si (e.g., viatunneling) is approximately 0.1-0.5 V/μm. In some cases, dopants can beionized via impact of a free carrier while a sufficiently high currentdensity is driven through a semiconductor containing frozen dopants.This also facilitates increasing the density of free carriers.

In some embodiments, the propagation constant of the waveguide ischanged by utilizing one or more of the DC Kerr effect and theFranz-Keldysh effect in addition to, or instead of, adjusting a densityof free carriers (e.g., electrons or holes) within the waveguide.Alternatively, the propagation constant of the waveguide may be changedby using one or more of: thermo-optic elements and stress-opticelements.

FIGS. 2A-2D, 3A-3D, 4A-4B, 5A-5B, 6A-6F, 7A-7B, and 8A-8B illustrateexample configurations of an optical switch device that provide the highspatial contrast as well as the high temporal contrast in the freecarrier density in accordance with some embodiments. In addition, theconfigurations illustrated in FIGS. 2A-2D, 3A-3D, 4A-4B, 5A-5B, 6A-6F,7A-7B, and 8A-8B are capable of switching operations even at lowtemperatures (e.g., a freeze-out temperature below 200 Kelvin).

FIGS. 2A and 2B are enlarged views of the coupling region 200 of theoptical switch device 100 shown in FIGS. 1A and 1B in accordance withsome embodiments.

The coupling region 200 includes the first waveguide 150 and the secondwaveguide 152. The first waveguide 150 is located adjacent to a firstdoped region 202-1 and a second doped region 204-1. The first dopedregion 202-1 is doped with dopants of a first type (e.g., p-typedopants, such as boron, gallium, and indium), and the second dopedregion 204-1 is doped with dopants of a second type (e.g., n-typedopants, such as phosphorus, arsenic, antimony, bismuth, and lithium) sothat a voltage applied between the first doped region 202-1 and thesecond doped region 204-1 increases the free carrier density in thefirst waveguide 150 (e.g., in region 210). In some cases, the voltage isapplied between a via 206-1 electrically connected to the first dopedregion 202-1 and a via 206-2 electrically connected to the second dopedregion 204-1.

In some embodiments, the first waveguide 150 is doped with dopants ofthe first type at a first dopant concentration and the first dopedregion 202-1 is doped with dopants of the first type at a second dopantconcentration that is higher than the first dopant concentration. Insome embodiments, the first waveguide 150 is doped with dopants of thesecond type at a third dopant concentration and the second doped region202-1 is doped with dopants of the second type at a fourth dopantconcentration that is higher than the third dopant concentration.

The first waveguide 150 has a region 210 (located between lines 212-1and 212-2) that is electrically coupled to the first doped region 202-1and a region 216 (located between lines 218-1 and 218-2) that iselectrically coupled to the second doped region 204-1, where the region210 and the region 216 are separated by at least a region 222 (locatedbetween lines 224-1 and 224-2). In some embodiments, the region 210 hasa width 214 that is greater than a width 226 of the region 222 so thatthe width of the optical waveguide 150 varies from the width 214 to thewidth 226 between the region 210 and the region 222, and the region 216has a width 220 that is greater than the width 226 of the region 222 sothat the width of the optical waveguide 150 varies from the width 220 tothe width 226 between the region 216 and the region 222. Compared to aconfiguration in which the first doped region 202-1 is directlyconnected to an optical waveguide without a width-varying region, thisconfiguration facilitates propagation of light within the opticalwaveguide 150 (e.g., by eliminating right angle corners that can causescattering of light). In some embodiments, the width 214 and the width220 are identical. In some embodiments, the width 214 is different fromthe width 220.

In some configurations, the second waveguide 152 has a structure that isa mirror image of the structure of the first waveguide 150. For example,the second waveguide 152 has same widths and heights as those of thefirst waveguide 150. In some cases, this symmetric configuration is usedfor coupling light between two waveguides made of a same material (e.g.,silicon, silicon nitride, silicon oxynitride, indium phosphide, galliumarsenide, aluminum gallium arsenide, lithium niobite, or any othersuitable photonic material including silicon and/or germanium basedmaterials). In some embodiments, the first waveguide 150 and the secondwaveguide 152 are made of different materials (e.g., the first waveguide150 is made of silicon and the second waveguide 152 is made of siliconnitride).

As explained above with respect to FIG. 1A, the material and dimensionsof the waveguides 150 and 152 and the surrounding region may be selectedto increase the coupling efficiency between the first waveguide 150 andthe second waveguide 152 while the optical switch device is in the “on”state. For brevity, such details are not repeated herein.

In some embodiments, the second waveguide 152 is located adjacent to athird doped region 202-2 and a fourth doped region 204-2. The thirddoped region 204-1 is doped with dopants of the first type, and thefourth doped region 204-2 is doped with dopants of the second type sothat a voltage applied between the third doped region 202-2 and thefourth doped region 204-2 increases the free carrier density in thesecond waveguide 152. This allows adjusting the carrier density in thesecond waveguide 152 separately (and sometimes independently) from thecarrier density in the first waveguide 150. In some cases, the voltageis applied between a via 206-3 electrically connected to the third dopedregion 202-2 and a via 206-4 electrically connected to the fourth dopedregion 204-2. In addition, the second waveguide 152 may have regions ofdifferent widths that correspond to respective regions of the firstwaveguide 150, which, in turn, facilitates the distribution of the freecarriers within the second waveguide 152. In some embodiments, thesecond waveguide 152 is not in electrical contact with any region dopedwith a same dopant concentration as the first doped region 202-1 or thesecond doped region 204-1. In some embodiments, the second waveguide 152is not in electrical contact with any doped region.

FIG. 2B illustrates structural elements located above the firstwaveguide 150 and the second waveguide 152, including lines 230, 232,234, and 236. In some embodiments, the lines 230, 232, 234, and 236 areformed in one or more metal layers, which may be formed in theback-end-of-line processing. Alternatively, the lines 230, 232, 234, and236 may be made of semiconductor materials, which may be formed in thefront-end-of-line processing. The line 230 is electrically coupled tothe first doped region 202-1 through the via 206-1 (shown in FIG. 2A)and the line 232 is electrically coupled to the second doped region204-1 through the via 206-2 (shown in FIG. 2A) so that the voltagebetween the line 230 and the line 232 is applied between the first dopedregion 202-1 and the second doped region 204-1. Similarly, the line 234is electrically coupled to the third doped region 202-2 through the via206-3 (shown in FIG. 2A) and the line 236 is electrically coupled to thefourth doped region 204-2 through the via 206-4 (shown in FIG. 2A) sothat the voltage between the line 234 and the line 236 is appliedbetween the third doped region 202-2 and the fourth doped region 204-2.

In some embodiments, a first voltage is applied between the lines 230and 232 while a second voltage different from the first voltage, such asa zero voltage or a non-zero voltage that is different from the firstvoltage, is applied between the lines 234 and 236 so that the freecarrier concentration in the first waveguide 150 is changed. In someimplementations, the first voltage provides a forward bias so that freecarriers are injected into the first waveguide 150, thereby increasingthe free carrier density and the absorption property value of the firstwaveguide 150. For example, for a configuration in which the first dopedregion 202-1 is doped with p-type dopants and the second doped region204-1 is doped with n-type dopants, applying a higher voltage (e.g., apositive voltage) to the first doped region 202-1 and applying a lowervoltage (e.g., a negative voltage) to the second doped region 204-1provides a forward bias.

FIG. 2B also illustrates a resistive heater 238 located above the firstwaveguide 150 and the second waveguide 152. In some cases, the resistiveheater 238 is a thin film resistor made of a resistive material (e.g.,tungsten, titanium nitride, tantalum nitride, amorphous silicon,silicides, such as tungsten silicide and nickel silicide, etc.). Theresistive heater 238 is coupled to power lines, such as a line 242 byone or more vias (e.g., via 240). When an electrical current flowsthrough the resistive heater 238, the resistive heater 238 generatesheat, which may be used to adjust the coupling ratio between the firstwaveguide 150 and the second waveguide 152 while the optical switchdevice 100 is in the “off” state (allowing the coupling of light betweenthe first waveguide 150 and the second waveguide 152).

Line AA′ represents a view from which the cross-section shown in FIG. 2Cis taken. As shown in FIG. 2C, the first waveguide 150 and the secondwaveguide 152 may be rib waveguides, where each rib waveguide has a ribregion 246 having the width 226 (corresponding to the width 226 of theregion 222 shown in FIG. 2A) located over a slab region 248 having thewidth 214 (corresponding to the width 220 of the region 216) that isgreater than the width 226. In some cases, the first waveguide 150having a rib waveguide configuration confines an optical mode of lightpropagating within the first waveguide 150 horizontally toward the ribregion 246. In some cases, this reduces an optical interaction betweenthe propagating light and the doped region 204-1. In some cases, thisfacilitates coupling of light between the first waveguide 150 and thesecond waveguide 152.

In FIG. 2C, the lines 230, 232, 234, and 236 are located within a samelayer (sometimes called a first metal layer or an M1 layer).Alternatively, the lines 230, 232, 234, and 236 may be located indifferent layers. For example, the lines 232 and 236 are located withinthe first metal layer and the lines 230 and 234 are located within athird metal layer (also called an M3 layer) while the line 242 islocated within a second metal layer (also called an M2 layer).

Although FIGS. 2A and 2B show only one region doped with the dopants ofthe first type (i.e., the first doped region 202-1) and only one regiondoped with the dopants of the second type (i.e., the second doped region204-1) adjacent to the first waveguide 150, multiple regions doped withthe dopants of the first type (e.g., regions 202-1 and 202-3) andmultiple regions doped with the dopants of the second type (e.g.,regions 204-1 and 204-3) may be located adjacent to the first waveguide150 as shown in FIG. 2D. Similarly, multiple regions doped with thedopants of the first type (e.g., regions 202-2 and 202-4) and multipleregions doped with the dopants of the second type (e.g., regions 204-2and 204-4) may be located adjacent to the second waveguide 152. Theregions doped with the dopants of the first type are interleaved withthe regions doped with the dopants of the second type (e.g., the region204-1 is located between the regions 202-1 and 202-3 and the region202-3 is located between the regions 204-1 and 204-3).

In some embodiments, the pitch 228 from the first doped region 202-1 tothe second doped region 204-1 is the same as the pitch 228 from thesecond doped region 204-1 to the region 202-3 and the pitch 228 from theregion 202-3 to the region 204-3. In some embodiments, the pitch 228 is40 μm or less, 20 μm or less, 10 μm or less, or 5 μm or less, althoughother pitches may be used. In some embodiments, the pitch 228 from thefirst doped region 202-1 to the second doped region 204-1 is differentfrom at least one of: a pitch from the second doped region 204-1 to theregion 202-3 and a pitch from the region 202-3 to the region 204-3.

FIGS. 3A and 3B are partial plan views of an optical switch device 300in accordance with some embodiments. The optical switch device 300 issimilar to the optical switch device 100 except that the two waveguides150 and 152 of the optical switch device 300 are stacked verticallywithin a stacked coupling region 400.

In addition to the portions 160-A and 170-A described above with respectto FIG. 1A, the optical waveguide 150 also includes a portion 160-B andthe optical waveguide also include a portion 170-B. The portion 160-B iscoupled to the portion 160-A via a portion 160-C located between lines102-3 and 102-1, and the portion 170-B is coupled to the portion 170-Avia a portion 170-C located between lines 102-4 and 102-1. In someembodiments, the portion 160-C is curved as shown in FIG. 3A. In someembodiments, the portion 160-C is straight. In some embodiments, theportion 170-C is curved as shown in FIG. 3A. In some embodiments, theportion 170-C is straight. In some embodiments, at least one of theportion 160-C and the portion 170-C is curved.

FIG. 3A also shows a portion 160-D of the first waveguide 150 and aportion 170-D of the second waveguide 152. The portion 160-D is coupledto the first portion 160-A via a portion 160-E located between lines102-2 and 102-5, and the portion 170-D is coupled to the portion 170-Avia a portion 170-E located between lines 102-2 and 102-6. In someembodiments, the portion 160-E is curved as shown in FIG. 3A. In someembodiments, the portion 160-E is straight. In some embodiments, theportion 170-E is curved as shown in FIG. 3A. In some embodiments, theportion 170-E is straight. In some embodiments, at least one of theportion 160-E and the portion 170-E is curved.

In some embodiments, at least one of the portions 160-C, 170-C, 160-E,and 170-E includes two or more curved sections (e.g., any of theportions 160-C, 170-C, 160-E, and 170-E can have two or more curvedsections having different centers of curvature, such as curved sectionsforming an s-curve). In some embodiments, the specific shape of thecurves is designed to ensure adiabaticity of the optical mode of lightas the light travels through the curved portion (e.g., light launchedinto the first curve in the fundamental mode will largely remain in thefundamental mode while propagating through the curves). As one ofordinary skill in the art would appreciate, the requirement foradiabaticity ensures that the excitation of higher order modes isreduced, e.g., excitation of higher order transverse modes, backscattered modes, and/or radiative modes, is minimized as the lighttravels through the curved sections. Depending on the geometricconstraints of the device layout, any number of different types ofcurves can be used including, e.g., Euler bends, Bezier curves, S-curvesand the like. Furthermore, the specific geometry that satisfies theadiabaticity condition will depend on the index of refraction around thewaveguide itself. Thus, the curve shape at the input portion (e.g., thecurve of a portion of the waveguide section 160-C proximate to line102-3) may be different from the curve at the output portion (e.g., thecurve of a portion of the waveguide section 160-C proximate to thecoupling region, just before the line 102-1). These curves may bedifferent because the presence of the other waveguide just above or justbelow may affect the local refractive index near the bend and therebychange the adiabaticity condition in that region.

As shown in FIG. 3A, light 110 injected into the portion 160-B of thefirst waveguide 150 propagates toward the portion 160-C and enters thefirst portion 160-A, where the light 110 is coupled to the portion 170-Awhile the absorption property of the portion 170-A is below a thresholdabsorption value, and subsequently, the light 110 propagates within thesecond waveguide 152 from the portion 170-A through the portion 170-Etoward the portion 170-D. Alternatively, the light 110 remains withinthe first waveguide 150 while the absorption property of the portion170-A is above the threshold absorption value and propagates from theportion 160-A through the portion 160-E toward the portion 160-D.

In some embodiments, at least one of the first waveguide 150 and thesecond waveguide 152 is a multi-mode waveguide. In some embodiments,both the first waveguide 150 and the second waveguide 152 are multi-modewaveguides. In slab or planar waveguides, some of the losses occur whentransmitted light comes into contact with walls that have irregularsurfaces. Planar waveguides fabricated with the currently availablesemiconductor fabrication techniques typically have top and bottomsurfaces that are smoother than side walls (e.g., the surface roughnessof the top and bottom surfaces is lower than the surface roughness ofthe side walls). The optical loss can decrease by reducing interactionbetween light propagating within the optical waveguide and the sidewalls. The disclosed embodiments include optical waveguides that arewide and short so that the distance between the side walls is greaterthan the distance between the top and bottom surfaces. Thisconfiguration reduces the interaction between the transmitted light andthe side walls. In particular, when a fundamental mode is transmittedthrough the wide and short optical waveguide, the fundamental mode has awidth that extends less toward the side walls of the optical waveguide,compared to a fundamental mode transmitted through a single modewaveguide. This, in turn, reduces the loss of the transmitted light. Insuch embodiments, the portions 160-A, 170-A, 160-B, 170-B, 160-C, 170-C,160-E, 170-E, 160-D, and 170-E may be portions of multi-mode waveguides.In some embodiments, a multi-mode waveguide is characterized by a widththat is greater than a height of the multi-mode waveguide.

FIG. 3B is similar to FIG. 3A, except that lines BB′ and CC′ areindicated in FIG. 3B. Line BB′ represents a view from which thecross-section shown in FIG. 3C is taken and line CC′ represents a viewfrom which the cross-section shown in FIG. 3D is taken.

Returning to FIG. 3A, the portion 160-B has a first lateral distance142, greater than a distance 140 (shown in FIG. 4A) between the portion160-A of the first waveguide 150 and the portion 170-A of the secondwaveguide 152, to the portion 170-B. As shown in FIG. 3C, the firstlateral distance 142 is an edge-to-edge distance between the portion160-B and the portion 170-B on a plane that is parallel to a surface 191of a substrate 190. In some embodiments, the first lateral distance 142is at least 1 μm, but one of ordinary skill in the art will appreciatethat this lateral distance depends on many factors including thewaveguide width, curve design, index of refraction of the waveguide coreand surrounding material, etc. The first lateral distance 142 betweenthe portion 160-B and the portion 170-B is significantly greater thanthe distance 140 between the first portion 160-A and the second portion160-B. As a result, light does not effectively couple between the thirdportion 160-B and the fourth portion 170-B.

Returning to FIG. 3A, the portion 160-D has a second lateral distance144, greater than the distance 140, to the portion 170-D. As shown inFIG. 3D, the second lateral distance 144 is an edge-to-edge distancebetween the portion 160-D and the portion 170-D on a plane that isparallel to the surface 191 of the substrate 190. In some embodiments,the second lateral distance 144 between the portion 160-D and theportion 170-D is identical to the first lateral distance 142 between theportion 160-B and the portion 170-B. In some embodiments, the secondlateral distance 144 between the portion 160-D and the portion 170-D isdifferent from the first lateral distance 142 between the portion 160-Band the portion 170-B.

FIG. 4A is an enlarged view of the stacked coupling region 400 of theoptical switch device shown in FIG. 3A in accordance with someembodiments.

In FIG. 4A, the stacked coupling region 400 includes the first waveguide150 and the second waveguide 152 that are stacked vertically.

FIG. 4A also shows a first doped region 402 and a second doped region404 that are located on opposite sides of the second waveguide 152 sothat a voltage applied between the first doped region 402 and the seconddoped region 404 increases the free carrier density in the secondwaveguide 152. In some cases, the voltage is applied between a via 406-1that is electrically coupled to the first doped region 402 and a via406-2 that is electrically coupled to the second doped region 404.

Line DD′ represents a view from which the cross-section shown in FIG. 4Bis taken. In FIG. 4B, the first waveguide 150 is located above thesecond waveguide 152. Alternatively, the second waveguide 152 may belocated above the first waveguide 150.

As shown in FIG. 4B, the second waveguide 152 may be a rib waveguidewith a rib region 446 having the width 426 located over a slab region448 having the width 420 that is greater than the width 426. The width422 of the first waveguide 150 is the same as the width 426 of the ribregion 446. Alternatively, the width 422 of the first waveguide 150 maybe different from the width 426 of the rib region 446. In FIG. 4B, thedash lines indicating the rib region 446 and the slab region 448 areoffset from the boundaries of the rib region 446 and the slab region 448for clarity.

The first doped region 402 is electrically connected to a line 432through the via 406-1 and the second doped region 404 is electricallyconnected to a line 436 through the via 406-2 so that the voltagebetween the line 432 and the line 436 is applied between the first dopedregion 402 and the second doped region 404.

FIG. 5A is an enlarged view of the stacked coupling region 400 of theoptical switch device shown in FIG. 3A in accordance with some otherembodiments.

The stacked coupling region 400 shown in FIG. 5A is similar to thestacked coupling region 400 shown in FIG. 4A except that the secondwaveguide 152 in FIG. 5A is a planar ribbed waveguide with regions 502(e.g., regions 502-1 through 502-4) having a width 524 interleaved withregions 504 (e.g., regions 504-1 through 504-5) having a width 526 thatis different from the width 524. At least one of the width 524 and thewidth 526 is different from the width 522 of the first waveguide 150. Insome configurations, both the width 524 and the width 526 are differentfrom the width 522. A respective region 504 has a length 556, which maybe less than 1 μm, less than 2 μm, less than 3 μm, less than 4 μm, lessthan 5 μm, less than 6 μm, less than 7 μm, less than 8 μm, less than 9μm, less than 10 μm, between 100 nm and 1 μm, between 500 nm and 2 μm,between 1 μm and 3 μm, between 2 μm and 4 μm, between 3 μm and 5 μm,between 4 μm and 6 μm, between 5 μm and 7 μm, between 6 μm and 8 μm,between 7 μm and 9 μm, between 8 μm and 10 μm, although the respectiveregion 504 may have a different length. The regions 504 have a pitch558, which may be less than 1 μm, less than 2 μm, less than 3 μm, lessthan 4 μm, less than 5 μm, less than 6 μm, less than 7 μm, less than 8μm, less than 9 μm, less than 10 μm, between 100 nm and 1 μm, between500 nm and 2 μm, between 1 μm and 3 μm, between 2 μm and 4 μm, between 3μm and 5 μm, between 4 μm and 6 μm, between 5 μm and 7 μm, between 6 μmand 8 μm, between 7 μm and 9 μm, or between 8 μm and 10 μm, although therespective region 504 may have a different pitch.

Line EE′ represents a view from which the cross-section shown in FIG. 5Bis taken. FIG. 5B is similar to FIG. 4B except that (i) the firstwaveguide 150 has a thickness 528 that is different from the thickness530 of the second waveguide 152, and (ii) the width 522 is differentfrom the width 526 as shown in FIG. 5A. In some implementations, thefirst waveguide 150 has a thickness that is the same as the thickness528 of the second waveguide 152.

FIG. 5B also shows that a portion, having the width 526, of a rib region546 of the second waveguide 152 is located over a slab region 548 havingthe width 520 that is greater than the width 526. In FIG. 5B, the dashlines indicating the rib region 546 and the slab region 548 are offsetfrom the boundaries of the rib region 546 and the slab region 548 forclarity.

A ribbed waveguide (e.g., a planar ribbed waveguide as shown in FIGS. 5Aand 5B or a vertical ribbed waveguide with alternating regions havingdifferent thicknesses) facilitates coupling of light between two opticalwaveguides having different refractive indices (e.g., a first waveguidemade of a first material having a first refractive index and a secondwaveguide made of a second material having a second refractive indexthat is different from the first refractive index, such as the firstwaveguide made of silicon nitride having a refractive index of 1.9 andthe second waveguide made of silicon having a refractive index of 3.48).

FIGS. 6A and 6E are enlarged views of the stacked coupling region 400 ofthe optical switch device shown in FIG. 3A in accordance with yet someother embodiments.

The stacked coupling region 400 shown in FIG. 6A is similar to thestacked coupling region shown in FIG. 5A at least in that the secondwaveguide 152 is a planar ribbed waveguide with interleaving regions 602and 604 of different widths. However, the stacked coupling region 400shown in FIG. 6A has a plurality of separate regions 612 (e.g., regions612-1 through 612-4) doped with dopants of a first type and a pluralityof separate regions 614 (e.g., regions 614-1 through 614-4) doped withdopants of a second type that is different from the dopants of the firsttype. Each region 612 is separate from the rest of the regions 612, andeach region 614 is separate from the rest of the regions 614.

In FIG. 6A, regions 602 (e.g., regions 602-1 through regions 602-4) andregions 604 (e.g., regions 604-1 through 604-5) are offset from thedoped regions 612 and 614. For example, the region 604-2 spans over thecombination of regions 612-1 and 614-1 and the combination of regions612-2 and 614-2 so that a portion of the region 604-2 is located withina combination of the doped regions 612-1 and 614-1 and a differentportion of the region 604-2 is located within a combination of the dopedregions 612-2 and 614-2. Similarly, a portion of the region 604-3 islocated within a combination of the doped regions 612-2 and 614-2 and adifferent portion of the region 604-3 is located within a combination ofthe doped regions 612-3 and 614-3. A portion of the region 604-4 islocated within a combination of the doped regions 612-3 and 614-3 and adifferent portion of the region 604-4 is located within a combination ofthe doped regions 612-4 and 614-4. A portion of the region 604-5 islocated within a combination of the doped regions 612-4 and 614-4.

FIG. 6A also shows that the combination of doped regions 612-1 and 614-1is located separately from the combination of doped regions 612-2 and614-2, which is also located separately from the combination of dopedregions 612-3 and 614-3. For example, a region 618 located between thecombination of doped regions 612-1 and 614-1 and the combination ofdoped regions 612-2 and 614-2 is either undoped, or doped with dopantsat a dopant concentration that is different from the dopantconcentration of the doped region 612-1 or the doped region 614-1 (e.g.,the dopant concentration of the region 618 is at least 10 times, 100times, or 1000 times less than the dopant concentration of the dopedregion 612-1 or the doped region 614-1). The combination of regions612-3 and 614-3 is also located separately from the combination ofregions 612-4 and 614-4.

Line FF′ represents a view from which the cross-section shown in FIG. 6Bis taken. Line GG′ represents a view from which the cross-section shownin FIG. 6C is taken. Line HH′ represents a view from which thecross-section shown in FIG. 6D is taken.

FIG. 6B is similar to FIG. 5B except that the entire rib region 646 andthe entire slab region 648 are doped. For example, a portion of the ribregion 646 and the slab region 648 is doped with dopants of the firsttype, and the rest of the rib region 646 and the slab region 648 isdoped with dopants of the second type.

FIG. 6C is similar to FIG. 6B except that the rib region 646 has thewidth 524 that is different from the width 526 of the rib region 646shown in FIG. 6B.

FIG. 6D is similar to FIG. 6B except that the rib region 646 and theslab region 648 are not doped (or doped at a dopant concentration thatis lower than the dopant concentration of the region 612-1 or the region614-1).

This configuration allows a rapid change in the free carrier density inthe second waveguide 152 by facilitating the movement of free carriersaway from the intersection between two adjoining doped regions dopedwith dopants of different types (e.g., the intersection between thedoped regions 612-1 and 614-1).

FIG. 6E illustrates structural elements located above the firstwaveguide 150 and the second waveguide 152, including lines 630, 632,634, and 636. The line 634 is electrically coupled to the doped region614-2 through the via 606-3 (shown in FIG. 6F) and the line 636 iselectrically coupled to the doped region 612-2 through the via 606-4(shown in FIG. 6F) so that the voltage between the line 634 and the line636 is applied between the doped region 614-2 and the doped region612-2. In addition, the line 634 is electrically coupled to the dopedregion 614-4 through the via 606-7 and the line 636 is electricallycoupled to the doped region 612-4 through the via 606-8 so that the samevoltage between the line 634 and the line 636 is applied between thedoped region 614-4 and the doped region 612-4. Similarly, the line 630is electrically coupled to the doped region 612-1 through the via 606-1and the line 632 is electrically coupled to the doped region 614-1through the via 606-2 so that the voltage between the line 630 and theline 632 is applied between the doped region 612-1 and the doped region614-1. In addition, the line 630 is electrically coupled to the dopedregion 612-3 through the via 606-5 and the line 632 is electricallycoupled to the doped region 614-3 through the via 606-6 so that the samevoltage between the line 630 and the line 632 is applied between thedoped region 612-3 and the doped region 614-3. In some implementations,the applied voltage provides a reverse bias so that free carriers moveaway from the junction between a respective region 612 and an adjoiningregion 614, thereby forming a depletion region and reducing the freecarrier density and the absorption property value of the secondwaveguide 152. For example, for a configuration in which the regions 612are doped with p-type dopants and the regions 614 are doped with n-typedopants, applying a lower voltage (e.g., a negative voltage) to theregions 612 and applying a higher voltage (e.g., a positive voltage) tothe regions 614 provides a reverse bias.

Line II′ represents a view from which the cross-section shown in FIG. 6Fis taken. FIG. 6F is similar to FIG. 6C except that FIG. 6F shows thelines 630, 632, 634, and 636 and vias 606-5, 606-6, 606-3, and 606-4connecting the lines 630, 632, 634, and 636 to respective doped regions.

FIG. 7A is an enlarged view of the coupling region of the optical switchdevice shown in FIG. 3A in accordance with some embodiments. FIG. 7A issimilar to FIG. 6A except that the regions 612 doped with the dopants ofthe first type are located separately from the regions 614 doped withthe dopants of the second type. For example, the doped region 612-1 inFIG. 7A is located separately from the doped region 614-1, whereas thedoped region 612-1 in FIG. 6A is located in contact with the dopedregion 614-1. In particular, FIG. 7A shows a region 620 extending alonga length-wise direction 622 of the second waveguide 152 and locatedbetween a respective region 612 doped with the dopants of the first typeand an adjacent region 614 doped with the dopants of the second type.The region 620 is either undoped, or doped with dopants at a dopantconcentration that is different from the dopant concentration of thedoped region 612-1 or the doped region 614-1 (e.g., the dopantconcentration of the region 618 is at least 10 times, 100 times, or 1000times less than the dopant concentration of the doped region 612-1 orthe doped region 614-1). In some implementations, the regions 618 and620 have the same dopant concentration.

Line JJ′ represents a view from which the cross-section shown in FIG. 7Bis taken. FIG. 7B is similar to FIG. 6B except that the region 614-4 andthe region 612-2 are separated by the region 620.

Although various lines and vias for providing a voltage between a region614 doped with the dopants of the first type and a region 612 doped withthe dopants of the second type are not shown in FIGS. 7A and 7B, aperson having ordinary skill in the art would understand that the lines(e.g., lines 630, 632, 634, and 636) and vias 606 shown in FIGS. 6E and6F may be used to provide a voltage between the region 614 and theregion 612. For brevity, such details are not repeated herein.

Although FIGS. 5A-5B, 6A-6F, and 7A-7B illustrate that the secondwaveguide 152 as a planar ribbed waveguide, in some implementations, thesecond waveguide 152 is a vertical ribbed waveguide. Alternatively, thesecond waveguide 152 may be a strip waveguide.

FIGS. 8A and 8B illustrate optical switch devices in accordance withsome embodiments, in which a number of input ports is different from anumber of output ports.

The optical switch device 800 shown in FIG. 8A is similar to the opticalswitch device 300 shown in FIG. 3A. However, the optical switch device800 differs from the optical switch device 300 at least in that thesecond waveguide 152 is not directly coupled to an input port of theoptical switch device 800. Instead, one end of the second waveguide 152in FIG. 8A is located adjacent to the coupling region 400 so that thesecond waveguide 152 is configured to conditionally receive light fromthe first waveguide 150 (based on the absorption property of a portionof the second waveguide 152 in the coupling region 400). Light injectedinto the first waveguide 150 propagates toward, and enters, the firstportion 160-A, where the light is coupled to the portion 170-A while theabsorption property of the portion 170-A is below a threshold absorptionvalue, and subsequently, the light propagates within the secondwaveguide 152 from the portion 170-A through the portion 170-E towardthe portion 170-D. Alternatively, the light remains within the firstwaveguide 150 while the absorption property of the portion 170-A isabove the threshold absorption value and propagates from the portion160-A through the portion 160-E toward the portion 160-D.

The one end of the second waveguide 152 located adjacent to the couplingregion 400 may be tapered, which facilitates coupling of the light fromthe first waveguide 150 to the second waveguide 152 while the absorptionproperty of the portion 170-A is below the threshold absorption value.

FIG. 8B shows an optical switch device 820 that is similar to theoptical switch device 800 shown in FIG. 8A. However, in the opticalswitch device 820, neither the first waveguide 150 nor the secondwaveguide 152 is directly coupled to an input port of the optical switchdevice 820. Instead, the optical switch device 820 includes a waveguide850 that is optically coupled to the second waveguide 152 so that lightpropagating within the waveguide 850 is coupled to the second waveguide152. One end of the first waveguide 150 in FIG. 8B is located adjacentto the coupling region 400 so that the first waveguide 150 is configuredto conditionally receive light from the second waveguide 152 (based onthe absorption property of a portion of the first waveguide 150 in thecoupling region 400). Light coupled into the second waveguide 152 fromthe waveguide 850 propagates toward, and enters, the first portion170-A, where the light is coupled to the portion 160-A while theabsorption property of the portion 160-A is below a threshold absorptionvalue, and subsequently, the light propagates within the first waveguide150 from the portion 160-A through the portion 160-E toward the portion160-D. Alternatively, the light remains within the second waveguide 152while the absorption property of the portion 160-A is above thethreshold absorption value and propagates from the portion 170-A throughthe portion 170-E toward the portion 170-D.

FIG. 8B also shows that the second waveguide 152 is located above thefirst waveguide 150. However, a person having ordinary skill in the artwould understand that the second waveguide 152 may be located below thefirst waveguide 150 as shown in FIG. 8A. For brevity, such details arenot repeated herein.

The optical switch devices 800 and 820 are 1×2 optical switch devices(each having one input port and two output ports), unlike the opticalswitch device 300, which is a 2×2 optical switch device (having twoinput ports and two output ports). A person having ordinary skill in theart would understand that an optical switch device with a differentnumber of input ports and/or a different number of output ports may bemade and used based on the information provided herein. For example, acascaded optical switch device having one input port and more than twooutput ports may be made and operated. For brevity, such details are notrepeated herein.

FIG. 9 is a flowchart illustrating method 900 of operating an opticalswitch device in accordance with some embodiments.

The method 900 includes (902) transmitting light into the firstsemiconductor structure of any optical switch device described hereinwhile a first voltage satisfying a first voltage condition is appliedbetween the first doped region and the second doped region for couplingthe light from the first waveguide to the second waveguide. In someembodiments, the first voltage condition requires that the appliedvoltage is below a first voltage threshold.

In some embodiments, the method 900 also includes, prior to, orsubsequent to, coupling the light from the first waveguide to the secondwaveguide, (904) transmitting the light into the first semiconductorstructure while a second voltage satisfying a second voltage conditiondifferent from the first voltage condition is applied between the firstdoped region and the second doped region for propagating the lightwithin the first waveguide without coupling the light from the firstwaveguide to the second waveguide. In some embodiments, the secondvoltage condition is that the second voltage does not satisfy the firstvoltage condition. Alternatively, the second voltage condition is thatthe applied voltage is above a second voltage threshold, which may ormay not be the same as the first voltage threshold.

For example, for an optical switch device with the coupling region 200illustrated in FIG. 2A, the first voltage condition requires that thefirst voltage does not provide a forward bias (e.g., the first voltageis below a forward bias voltage threshold so that the first voltage doesnot provides the forward bias). Thus, applying the first voltage betweenthe first doped region and the second doped region does not causeinjection of free carriers into the optical waveguide located adjacentto the first doped region and the second doped region, therebymaintaining the free carrier density low (e.g., 4×10¹⁶ cm⁻³ or less, forexample). For such optical switch devices, the second voltage conditionrequires that the second voltage provides a forward bias (e.g., thesecond voltage is above the forward bias voltage threshold). Thus,applying the second voltage between the first doped region and thesecond doped region causes injection of free carriers into the opticalwaveguide located adjacent to the first doped region and the seconddoped region, thereby increasing the free carrier density (e.g., 6×10¹⁸cm⁻³ or less, for example).

In another example, for an optical switch with the coupling region 400illustrated in FIG. 6A, the first voltage condition requires that thefirst voltage provides a reverse bias (e.g., the first voltage is lessthan a reverse bias voltage threshold so that the first voltage providesthe reverse bias). Thus, applying the first voltage between the firstdoped region and the second doped region causes formation (or anenlargement) of a depletion region between the first doped region andthe second doped region, thereby reducing the free carrier density(e.g., 4×10¹⁶ cm⁻³ or less, for example). For such optical switchdevices, the second voltage condition requires that the second voltagedoes not provide a reverse bias (e.g., the second voltage is above thereverse bias voltage threshold). Thus, applying the second voltagebetween the first doped region and the second doped region does notcause formation of the depletion region between the first doped regionand the second doped region, thereby increasing the free carrier density(e.g., 6×10¹⁸ cm⁻³ or less, for example).

In some embodiments, the second semiconductor structure has (906) afirst carrier density while the first voltage is applied between thefirst doped region and the second doped region, and the secondsemiconductor structure has a second carrier density that is greaterthan the first carrier density by a factor of at least 100 while thesecond voltage is applied between the first doped region and the seconddoped region.

In some embodiments, the light is coupled (908) from the first waveguideto the second waveguide while the optical switch device is at atemperature between 40 Kelvin and 200 Kelvin. The free carrier densitygenerally decreases at lower temperatures. However, the optical switchdevices described herein allow more effective changes in the absorptionproperty value of the waveguide, and such optical switch devices canprovide switching operations even at a low temperature, such as atemperature below 200 Kelvin, a temperature below 150 Kelvin, atemperature below 100 Kelvin, a temperature below 90 Kelvin, atemperature below 80 Kelvin, a temperature below 70 Kelvin, atemperature below 60 Kelvin, a temperature below 50 Kelvin, atemperature below 45 Kelvin, or a temperature at 40 Kelvin. Thus, theoptical switch devices described herein can be used even for opticalapplications requiring switching operations at cryogenic temperatures(e.g., less than 93 Kelvin).

In some embodiments, applying the second voltage between the first dopedregion and the second doped region while the optical switch device is ata temperature less than 40 Kelvin foregoes switching the optical switchdevice between the “on” state and the “off” state. For example, in someembodiments, applying the second voltage between the first doped regionand the second doped region while the optical switch device is at atemperature less than 40 Kelvin allows (910) coupling of the light fromthe first waveguide to the second waveguide. As explained above, thefree carrier density decreases at low temperatures. At a temperatureless than 40 Kelvin for example, the free carrier density will decreasesignificantly so that applying the second voltage may not providesufficient increase in the free carrier density, thereby interferingwith the switching operations of the optical switch device. In someembodiments, applying the second voltage between the first doped regionand the second doped region while the optical switch device is at atemperature less than 35 Kelvin, 30 Kelvin, 25 Kelvin, or 20 Kelvinforegoes transitioning the optical switch device between the “on” stateand the “off” state.

The terminology used in the description of the various describedembodiments herein is for the purpose of describing particularembodiments only and is not intended to be limiting. As used in thedescription of the various described embodiments and the appendedclaims, the singular forms “a”, “an” and “the” are intended to includethe plural forms as well, unless the context clearly indicatesotherwise. It will also be understood that the term “and/or” as usedherein refers to and encompasses any and all possible combinations ofone or more of the associated listed items. It will be furtherunderstood that the terms “includes,” “including,” “comprises,” and/or“comprising,” when used in this specification, specify the presence ofstated features, integers, steps, operations, elements, and/orcomponents, but do not preclude the presence or addition of one or moreother features, integers, steps, operations, elements, components,and/or groups thereof.

As used herein, the term “if” is, optionally, construed to mean “when”or “upon” or “in response to determining” or “in response to detecting”or “in accordance with a determination that,” depending on the context.

The foregoing description, for purpose of explanation, has beendescribed with reference to specific embodiments. However, theillustrative discussions above are not intended to be exhaustive or tolimit the scope of the claims to the precise forms disclosed. Manymodifications and variations are possible in view of the aboveteachings. For example, although FIGS. 6A-6F illustrate the opticalswitch device in which two optical waveguides have different widths, aperson having ordinary skill in the art would understand that the twooptical waveguides can have a same width in a manner analogous to theoptical waveguides shown in FIG. 4B. The embodiments were chosen inorder to best explain the principles underlying the claims and theirpractical applications, to thereby enable others skilled in the art tobest use the embodiments with various modifications as are suited to theparticular uses contemplated.

What is claimed is:
 1. An optical switch device, comprising: a firstsemiconductor structure configured to operate as a first waveguide; anda second semiconductor structure configured to operate as a secondwaveguide, the second semiconductor structure being located above orbelow the first semiconductor structure and separated from the firstsemiconductor structure, the second semiconductor structure including afirst portion having a first width and a second portion having a widthdifferent from the first width and located on the first portion, thefirst portion being located between a first doped region and a seconddoped region so that the optical switch device is capable of couplinglight propagating in the first waveguide to the second waveguide basedon a voltage applied between the first doped region and the second dopedregion.
 2. The optical switch device of claim 1, wherein: the firstsemiconductor structure and the second semiconductor structure areconfigured to couple light propagating in the first waveguide to thesecond waveguide while a first voltage satisfying a first voltagecondition is applied between the first doped region and the second dopedregion; and the first semiconductor structure and the secondsemiconductor structure are configured to forego coupling of the lightpropagating in the first waveguide to the second waveguide while asecond voltage satisfying a second voltage condition different from thefirst voltage condition is applied between the first doped region andthe second doped region.
 3. The optical switch device of claim 2,wherein: the second semiconductor structure has a first carrier densitywhile the first voltage is applied between the first doped region andthe second doped region; and the second semiconductor structure has asecond carrier density that is greater than the first carrier density bya factor of at least 100 while the second voltage is applied between thefirst doped region and the second doped region.
 4. The optical switchdevice of claim 2, wherein: the first portion has a first absorptionproperty while the first voltage is applied between the first dopedregion and the second doped region and a second absorption property thatis different from the first absorption property while the second voltageis applied between the first doped region and the second doped region.5. The optical switch device of claim 1, wherein: the second portionincludes a plurality of first sections having a second width interleavedby a plurality of second sections having a third width different fromthe second width.
 6. The optical switch device of claim 5, wherein: eachfirst section of the plurality of first sections has a first length; andeach second section of the plurality of second sections has a secondlength that is different from the first length.
 7. The optical switchdevice of claim 1, wherein: the second portion includes a plurality offirst sections having a first thickness interleaved by a plurality ofsecond sections having a second thickness different from the firstthickness.
 8. The optical switch device of claim 1, wherein: the firstsemiconductor structure is made of a first semiconductor material havinga first index of refraction; and the second semiconductor structure ismade of a second semiconductor material having a second index ofrefraction that is different from the first index of refraction.
 9. Theoptical switch device of claim 1, wherein: the first doped region isdoped with donor dopants; and the second doped region is doped withacceptor dopants.
 10. The optical switch device of claim 1, wherein: oneof the first waveguide and the second waveguide is connected to an inputport of the optical switch device for receiving light; the firstwaveguide is connected to a first output port of the optical switchdevice; and the second waveguide is connected to a second output port ofthe optical switch device that is different from the first output portof the optical switch device.
 11. A method, comprising: transmittinglight into the first semiconductor structure of the optical switchdevice of claim 1 while a first voltage satisfying a first voltagecondition is applied between the first doped region and the second dopedregion for coupling the light from the first waveguide to the secondwaveguide.
 12. The method of claim 11, further comprising: prior to, orsubsequent to, coupling the light from the first waveguide to the secondwaveguide, transmitting the light into the first semiconductor structurewhile a second voltage satisfying a second voltage condition differentfrom the first voltage condition is applied between the first dopedregion and the second doped region for propagating the light within thefirst waveguide without coupling the light from the first waveguide tothe second waveguide.
 13. The method of claim 11, wherein: the light iscoupled from the first waveguide to the second waveguide while theoptical switch device is at a temperature between 40 Kelvin and 200Kelvin.
 14. The method of claim 11, wherein: applying the second voltagebetween the first doped region and the second doped region while theoptical switch device is at a temperature less than 40 Kelvin allowscoupling of the light from the first waveguide to the second waveguide.